This is a proposal to determine the three dimensional structure and the amino acid sequence of two crystalline Fab fragments (Fab New and Fab Hil) from human myeloma immunoglobulins. The amino acid sequences will be correlated with the Fourier maps of the electron density in the crystals to provide a three dimensional model of the immunoglobulin fragments. X-ray diffraction techniques have been used to obtain a 2.8 Angstrom Units-resolution Fourier map of one of these fragments, Fab New. A 2,0 Angstrom Units-resolution Fourier map will be calculated in the near future. Amino acid sequence data will be necessary for the complete, unequivocal interpretation of these Fourier maps and also, to locate immunologically important regions of the sequence in the three dimensional structure. It is proposed to carry out similar X-ray diffraction and sequence studies on Fab Hil and to compare its structure with that of Fab New.